Gamma radiation exposures from nuclear weapons raises the temperature of inertial instruments. Certain kinds of inertial instruments, most notably Vibrating Beam Accelerometers (VBAs), exhibit measurement errors due to not just this change in temperature, but also the rate of change of temperature (T-dot).
High accuracy inertial instruments are temperature controlled to eliminate errors due to temperature drift. In many kinds of instruments T-dot induces greater errors than does the absolute temperature change. Since it is impossible to prevent an initial sudden rate of temperature change due to pulsed gamma ray exposure, the usual approach is to try, when possible, to rapidly return the instrument to its set point temperature. However, the rate of change of temperature while returning the unit to the set point may induce errors greater than those resulting from the absolute change of temperature.
As shown in FIG. 2-1, consider a typical VBA 24 with a proof mass 30 attached to a mechanically stable member 32 by some kind of elastic hinge 34. Heating due to radiation is quite different. Radiation causes consistent heating throughout the proof mass 30 and stable member 32. Since the proof mass and stable member are usually made from the same material, the temperature increase throughout the two masses will be very nearly equal with the major difference being due to unequal exposures of the two masses. The small gradients would produce small strains with a consequently small error. When the unit is thermally stabilized (post thermal increase event) there are no temperature gradients across the hinge 34 (FIG. 2-2). When the temperature controller attempts to lower the temperature of the instrument, initially, heat will begin to flow out of the stable member 32 and a temperature gradient will form across the length of the device (x axis). Then, after some delay, heat will begin to flow through the hinge 34 creating a significant gradient. This gradient will continue to exist while the heat is removed from the proof mass 30 (see FIG. 2-3). Since the hinge 34 has a relatively small cross-sectional area, the process can take quite a while. During this time, the thermal gradients across the hinge 34 will result in a change in the strains within the hinge 34 and thereby inducing errors into the movement of the proof mass 30.